Former White House adviser Karl Rove defied a congressional subpoena and refused to testify yesterday about allegations of political pressure at the Justice Department, including whether he influenced the prosecution of a former Democratic governor of Alabama.

Rep. Linda Sanchez, chairman of a House subcommittee, ruled with backing from fellow Democrats on the panel that Mr. Rove was breaking the law by refusing to cooperate — perhaps the first step toward holding him in contempt of Congress.

The White House has cited executive privilege as a reason he and others who serve or served in the administration should not testify, arguing that internal administration communications are confidential and that Congress cannot compel officials to testify. Mr. Rove says he is bound to follow the White House’s guidance, although he has offered to answer questions specifically on the Siegelman case — but only with no transcript taken and not under oath.

Lawmakers subpoenaed Mr. Rove in May in an effort to force him to talk about whether he played a role in prosecutors’ decisions to pursue cases against Democrats, such as Governor Siegelman of Alabama, or in firing federal prosecutors considered disloyal to the Bush administration.

Rove had been scheduled to appear at the House Judiciary subcommittee hearing yesterday morning. A placard with his name sat in front of an empty chair at the witness table, with a handful of protesters behind it calling for Rove to be arrested.